The steep rises in price of crude and gas have led operators to increase their production, much of it through secondary recovery. The increase in production has resulted in increases in water cut and H2S which, together with other corrosive elements such as CO2 and chlorides, have caused serious concerns regarding safety and corrosion. The traditionally used steel OCTG suffer failures within a relatively short period of operation in these environments. Therefore the use of corrosion resistant alloys (CRA) is set to increase over the coming years. The cost difference between the lower and higher end of CRA can be very substantial. The traditional material selection approach has heavily depended on the use of standards and desktop corrosion rate predictions. However the confidence level with this approach is limited. A comprehensive and innovative program has therefore been adopted to address this issue. Our approach has been two fold: first we designed and manufactured a special downhole coupon holder in order to expose a number of different CRA material in a selected number of our reservoirs. Secondly, we carried out a comprehensive program of corrosion tests on a range of materials in a variety of our reservoir conditions simulated in the laboratory. The combination of field, lab and desk-top evaluation enables us to select the most technically suitable and cost effective material for each specific environment.
A total of 14 alloys from a number of manufacturers have been tested. These ranged from the L-80 carbon steel (used as reference) to the lower end CRA such as 13Cr and Super 13Cr to mid range CRA such as duplex stainless steel ton the higher end nickel-chromium alloys. The results have been very encouraging. Ten out of 14 materials tested exhibited insignificant levels of general and localized corrosion (pitting and crevice). The four which suffered corrosion higher than this level were the L-80 carbon steel reference material and most of the 13Cr alloys. Encouragingly, all the Super 13Cr alloys passed all the corrosion tests in all the environments satisfactorily. Even in the environmental cracking tests the super 13Cr alloys were successful in the majority of the tested environments. These are at the cheaper end of CRAs and using them instead of Ni-Cr alloys can make very substantial savings for the operating company, as well as proving a material which is technically far superior to the traditionally used L-80 carbon steel and provides a much longer operating life.
The field testing results were even more encouraging with the Super 13Cr alloys not suffering from cracking in any environments and, apart from the most aggressive one of the eight field testing environments, they did not suffer from any significant corrosion or pitting.
A classification of the tested alloys by test condition, based on the laboratory results is presented. This shows that for every one of the downhole environments that we have tested, there are a range of acceptable materials available.